1. Field of the Invention
This invention relates to a separator plate suitable for use in various known types of fuel cells, such as molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte fuel cells, and phosphoric acid fuel cells. More particularly, this invention relates to a separator plate suitable for use in internally manifolded fuel cell stacks.
Generally, fuel cell electrical output units are comprised of a stacked plurality of individual cells separated by inert or bi-polar electronically conductive ferrous metal separator plates. Individual cells are sandwiched together and secured into a single stacked unit to achieve desired fuel cell energy output. Each individual cell generally includes an anode and cathode electrode, a common electrolyte "tile" or "matrix" typically referred to as the active area components, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds to their respective reactant chambers between the separator plate and the active area components. The area of contact between the electrolyte and separator plate to maintain separation of the fuel and oxidant gases and prevent and/or minimize gas leakage is known as the wet seal. A major factor contributing to premature fuel cell failure is corrosion and fatigue in the wet seal area. This failure is hastened by thin-film electrochemical corrosion at stainless steel surfaces of the separator plate causing weakening of the wet seal structure through intracrystalline and transcrystalline cracking. Such failures permit undesired fuel and/or oxidant gas crossover and overboard gas leakage which interrupts the intended electrochemical oxidation and reduction reactions, thereby causing breakdown and eventual stoppage of cell current generation.
In molten carbonate fuel cells, in particular, under fuel cell operating conditions, in the range of about 500.degree. C. to about 700.degree. C., molten carbonate electrolytes are very corrosive to ferrous metals which, due to their strength, are required for fuel cell housings and separator plates. The high temperature operation of stacks of molten carbonate fuel cells increases both the corrosion and thermal stress problems in the wet seal area, especially when the thermal coefficients of expansion of adjacent materials are different.
In addition to providing a wet seal structure between the separator plate and adjacent cell components, the separator plate also provides means for distributing fuel and oxidant gases to their respective reactant chambers. Known separator plates are comprised of a plurality of pieces welded, or otherwise secured, together to form the requisite wet seals and gas distribution means. In accordance with one embodiment of this invention, this invention provides fully internal manifolding of the fuel and oxidant gases to and from the individual cells of an assembled stack in a manner, due to the design of the cell components, in particular, the separator plate, which provides ease of assembly, long term endurance, stability of fuel cell operation, and reduced fabrication time and cost, in particular, for the separator plate.